Media Kit: Delete the PortPool in a more deliberate way

Letting it get deleted statically can cause segfaults since it is needed for
some final quit messages.

This mimic changes Axel made for the DormantNodeManager and
TimeSourceObjectManager.

I also pulled PortPool into its own file and header.

Fixes #15135.

Change-Id: Ie64753e1876d58b52f7cb95536c6be3df2e6d40c
Reviewed-on: https://review.haiku-os.org/c/haiku/+/1721
Reviewed-by: waddlesplash <waddlesplash@gmail.com>